In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipments (UEs)) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a controller node (such as a radio network controller (RNC) or a base station controller (BSC)) which supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). UTRAN is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. The 3GPP has developed specifications for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN). The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE). Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected to a core network (via Access Gateways, or AGWs) rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes (eNodeB's in LTE) and AGWs. As such, the radio access network (RAN) of an LTE system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
One result of the forum's work is the High Speed Downlink Packet Access (HSDPA) for the downlink, which was introduced in 3GPP WCDMA specification Release 5. HSDPA features a high speed channel (HSC) controller that functions, e.g., as a high speed scheduler by multiplexing user information for transmission over the entire HS-DSCH bandwidth in time-multiplexed intervals (called transmission time intervals (TTI)). Since HSDPA uses code multiplexing, several users can be scheduled at the same time.
The High Speed Downlink Packet Access (HSDPA) was followed by introduction of High Speed Uplink Packet Access (HSUPA) with its Enhanced Dedicated Channel (E-DCH) in the uplink in 3GPP WCDMA specification Release 6. E-DCH is dedicated uplink channel (from a user equipment (UE) to a Node-B) that has been enhanced for IP transmission Enhancements include using a short transmission time interval (TTI); fast hybrid ARQ (HARQ) between mobile terminal and the Node-B (with soft combining); scheduling of the transmission rates of mobile terminals from the Node-B. In addition, E-DCH retains majority of the features characteristic for dedicated channels in the uplink.
E-DCH comes with several channels from each UE. For example, the DPCCH carries pilot symbols and parts of the outband control signaling. Remaining outband control signalling for the enhanced uplink, e.g., scheduling requests, is carried on the E-DPCCH (E-Dedicated Physical Control Channel), while the E-DPDCH (E-Dedicated Physical Data Channel) carries the data transmitted using the enhanced uplink feature.
In 3GPP Rel-11, work is ongoing to improve the end user experience and performance especially in the CELL_FACH state. CELL_FACH is an RRC state in which the UE is known on cell level (i.e., has a cell id and a UE identifier assigned for such a cell), has a layer 2 connection, but does not have a dedicated physical layer resource. Instead, common physical layer resources are shared between users in CELL_FACH.
E-DCH is normally used as a dedicated channel in CELL_DCH state with one separate resource allocated per user. But E-DCH can also be used in CELL_FACH state by having a pool of E-DCH resources that can be temporarily assigned to a user in CELL_FACH. These resources are called common E-DCH resources. E-DCH resources are normally managed by the radio network controller (RNC), but the pool of common E-DCH resources is managed by a NodeB. The common E-DCH configurations are broadcasted to wireless terminals (UEs) in the cell.
The procedure to access the common E-DCH channel in CELL_FACH starts in the same way as Rel-99 RACH transmission, i.e., with preamble power ramping using randomly selected preamble signatures. Having detected the preamble, the NodeB acknowledges reception with an AICH sequence. The NodeB also informs the UE which common E-DCH resource it has assigned to the UE.                A common E-DCH resource is defined as:        Initial Serving Grant Value        E-DCH Transmission Time Interval        E-AGCH information        HARQ information        Uplink DPCH power control information        E-DPCCH information        E-DPDCH information        F-DPCH information        E-HICH information        UL scrambling code        HS-DPCCH parameters        
CELL_FACH is a state commonly used for battery and radio efficient use of radio resources for UEs in which data typically arrives in bursts with longer idle periods in between. Ideally, an UE should be inactive between the bursts but still be capable of swiftly moving into an active state when there are packets to send or receive. For this kind of on-off type traffic patterns, the connection set-up latency and signaling load has a significant impact both on the preservation of the device battery and on the transmission quality perceived by the end user.
Information about a network and a serving cell is broadcast to all wireless terminals in a number of system information blocks (SIB5). E-DCH resource configurations are broadcasted in System Information Block 5 (SIB5). Some of the broadcasted parameters are common for all common E-DCH resources, for instance the TTI (Time Transmission Interval) configuration.
One of the enhancements for standardization in Rel 11 was the stand-alone HS-DPCCH. Stand-alone HS-DPCCH means that the network can command the UE to request a common E-DCH resource. When the UE receives the command, the UE requests a common E-DCH resource and the network may allocate it. If the UE does not have any data to transmit, the UE only sends only the DPCCH during the synchronization procedure AA. Synchronization procedure AA may be used when one downlink F-DPCH and uplink dedicated physical channels are to be set up on a frequency as a consequence of an Enhanced Uplink in CELL_FACH procedure. Synchronization procedure AA is used to synchronize the UL of the UE with the Cell, and is a faster procedure than the traditional synchronization because the UE maintains certain sync during the Random Access. Synchronization procedure AA is explained, e.g., in 3GPP TS 25.214. Physical layer procedures (FDD). V.10.6.0, section 4.3.2.3A. Afterwards the UE will transmit the DPCCH, the HS-DPCCH, and will transmit the E-DPDCH and E-DPCCH to convey the Scheduling Information to the network for collision resolution purposes. In this case, the request of the common E-DCH resources is not due to UL data to be transmitted, but is initiated upon the network request.
The HS-DPCCH channel is used to aid the DL transmissions on HS-DSCH by providing ACK/NACK and CQI reports to the Node B. Prior to Rel 8, the HS-DPCCH operated without any feedback in CELL_FACH. In Rel 8, the HS-DSCH could get this feedback if the UE was using a common E-DCH resource, in which case the HS-DPCCH was transmitted after content resolution until the UE released the E-DCH resource, e.g., when the UE had emptied the UL buffer and a timer known as the Tb timer had expired.
In Rel 11, the stand-alone HS-DPCCH can be requested by the Node B when there is DL data to the UE on HS-DSCH. This can be requested even if the UE has no UL data on E-DCH. The request is done by an HS-SCCH order to the UE, which upon reception starts the process of obtaining a common E-DCH resource using standard procedure, i.e., power ramping using a signature corresponding to a common E-DCH resource. When the UE receives an acknowledgement (ACK) it immediately starts to transmit the DPCCH and after a short synchronization period also the HS-DPCCH. The HS-DPCCH is transmitted until the E-DCH resource is released. A comparable action may be performed if a resource redirection (e.g. NACK in AI) but an index in E-AI is not configured, as long as there is no NACK in the E-AI.
There are two timers defined for the case of implicit release of the E-DCH, e.g., release done by the UE upon expiration of a timer. The first timer is the Ths-dpcch which is restarted after every reception of a downlink (DL) protocol data unit (PDU). The second timer is Tb, which is started when the uplink (UL) buffer is empty. The Tb timer is the reset every time there is new data in the UL buffer or when there is DL data received and restarted whenever the uplink buffer is empty again. The Tb is typically started for the first time after the UE has had at least one UL E-DCH transmission. In case of stand-alone HS-DPCCH there may not be any UL data since the E-DCH resource is requested to aid the DL transmissions. Hence, this is opposite of the typical case when an E-DCH requested for the purpose of transmitting UL data.
The Tb and Ths-dpcch timers expiration times are configured by the network. The Tb timer's expiration time is given by the “EDCH transmission continuation back off” and the expiration time for the Ths-dpcch is given by “HS-DPCCH transmission continuation back off”.
Preferably the Tb and Ths-dpcch timers are not run simultaneously. From the start the Ths-dpcch is used and once there is UL data, it is stopped and the Tb timer is used thereafter.
In order to accommodate a large number of users in CELL_FACH, the common E-DCH resources should not be kept longer than needed. This implies that the timer settings for Ths-dpcch and Tb should be as small as possible. In a possible cell configuration the Tb timer may be set to 0. This means that as soon as the UE has emptied its UL buffer, the E-DCH resource is released. This setting will be optimal to cater for UEs accessing the common E-DCH resource to transmit a short burst of UL data.
To aid DL HS-DSCH traffic the situation is different. The DL scheduling delay will cause jitter in the DL traffic and a larger timer setting is needed. As an example, a certain example product that can schedule up to 96 users (including CELL_DCH users), and with any reasonable scheduling algorithm it can be understood that there may be some time between the DL packets to any specific user. Also the CQI reports may be configured to be transmitted periodically on the HS-DPCCH which implicates that the resource should be kept long enough to allow for transmission of CQI reports.
The Tb timer takes over once there is UL data. This means that possibly two different timer settings will be used for the release of the E-DCH resource in the case of stand-alone HS-DPCCH. This also means that it will not be possible to have optimal system performance since optimization can occur either (1) for the UL data use case with a fast release (with bad consequences for the stand-alone HS-DPCCH) or (2) for the DL data case with a much slower release (with bad consequences for UL data use case).
A more severe problem is the fact that “E-DCH transmission continuation back off”, which is used to configure the Tb timer, is given in transmission time intervals (TTIs). This means that a specific configured value will result in different timer settings (measured in ms) depending on whether the UE is using a 2 ms TTI E-DCH resource or a 10 ms TTI E-DCH resource. For example, the highest available “E-DCH transmission continuation back off” is 80 TTIs, i.e., 160 ms for 2 ms TTI resource or 800 ms for a 10 ms TTI resource. In the case of stand-alone HS-DPCCH, the Tb timer takes over after the Ths-dpcch timer once there has been an UL transmission. This means that the Tb timer is used to assist the DL HS-DSCH with ACK/NACK and CQI reports. Since the DL is unaffected by the choice of UL TTI, it is an undesirable situation to have the expiration time dependent on the TTI length of the UL E-DCH.
In case of Ths-dpcch, the optimal timer setting “HS-DPCCH transmission continuation back off” to assist DL HS-DSCH will be independent of the TTI length of the UL E-DCH. Once the Tb timer takes over, this is no longer possible to achieve.